A magnetic tunnel junction (MTJ) forms the basic memory element of a novel, non-volatile magnetic random access memory (MRAM) that promises high performance and endurance, and, moreover has the potential to be scaled to extremely small sizes. A magnetic tunnel junction (MTJ) is composed of a sandwich of two magnetic electrodes separated by an ultra-thin insulating layer. One of these layers forms the memory or storage layer and the other layer forms a reference layer whose magnetic structure is not changed during operation of the MRAM. Electrical current that tunnels between the reference and memory magnetic electrodes is spin-polarized: the magnitude of the spin-polarization is determined by a combination of the electronic properties of the magnetic electrodes and “spin-filtering” properties of the tunnel barrier.
In current-day MRAM the magnetic state of the MTJ, whether the magnetization of the electrode that forms the memory layer is oriented parallel or anti-parallel to that of the reference electrode, is changed by passing a current through the MTJ. The current, which is innately spin-polarized, delivers spin angular momentum, that once a threshold current is exceeded results in switching of the direction of the magnetic memory electrode moment. This transfer of spin angular momentum exerts a spin transfer torque (STT) and the magnetic memory electrode switched by this method is referred to as STT-MRAM. The magnitude of the switching current is reduced when the magnetization of the electrodes is oriented perpendicular to the layers. The magnitude of this current is limited by the size of the transistors used to provide the write current. This means that the thickness of the switchable magnetic electrode (memory electrode) must be sufficiently small that it can be switched by the available current. Thus for magnetization values of ˜1000 emu/cm3, the electrode must have a thickness that does not exceed approximately 1 nm.
The most promising materials that are being explored today for MTJs for dense MRAM comprise ferromagnetic electrodes formed from alloys of Co, Fe and B, and tunnel barriers formed from MgO (e.g. U.S. Pat. No. 8,008,097). The ferromagnetic electrodes are made sufficiently thin that the magnetizations of these electrodes are oriented perpendicular to these layers. The perpendicular magnetic anisotropy (PMA) of Co—Fe—B layers arises from the interfaces between these layers and the tunnel barrier and/or the underlayer on which the Co—Fe—B layer is deposited. Thus, these layers must be made sufficiently thin that the interface PMA overcomes the demagnetization energy that arises from the magnetic volume and increases in proportion with the magnetic volume of the Co—Fe—B layer. In practice, this means that the PMA is too weak to overcome thermal fluctuations when the device size is reduced to below ˜20 nm in size, since the thickness of the magnetic layer has to be below that required to maintain its moment perpendicular and below that needed to switch the magnetic layer with reasonable current densities. So far magnetic materials whose magnetic moments could be switched by STT in MRAM devices had either interfacial, shape, or no anisotropy. Such materials do not allow scaling of MRAM devices to sizes below ˜20 nm. What is needed are new materials for the ferromagnetic electrodes which display much larger PMA than that exhibited by Co—Fe—B and that preferably the PMA arises from the volume or bulk of the electrodes. A promising class of magnetic materials that has such a property are Heusler compounds. Heusler alloys1 are compounds with the chemical formula X2YZ or X′X″YZ wherein X and X′ and X″ and Y are transition metals or lanthanides (rare-earth metals) and Z is from a main group metal. The Heusler compounds have a structure of the type Cu2MnAl (defined in the Pearson Table) in which the elements are disposed on 4 interpenetrating face-centered cubic (fcc) lattices. Many compounds (˜800) are known in this family (T. Graf et. al., Progress in Sol. State Chem. 39, 1 (2011)). Some of these compounds are ferromagnetic or ferrimagnetic due to magnetic moments on the X and/or Y sites. Moreover, whilst the parent Heusler compounds are cubic and exhibit weak or no significant magnetic anisotropy, the structure of some of these compounds is found to be tetragonally distorted: due to this distortion the magnetization exhibited by these compounds may be preferably aligned along the tetragonal axis. Thus, thin films formed from such materials may exhibit PMA due to a magneto-crystalline anisotropy associated with their tetragonally distorted structure. Some known examples of such tetragonal Heusler compounds are Mn3Z where Z═Ga, Ge, Sn, and Sb or Mn2CoSn. The thickness of magnetic electrodes formed from Heusler alloys on Si/SiO2 substrates with use of conducting underlayers, to date, far exceeds the thickness of 1 nm. The thinnest layers to date are for the Heusler compound Mn3Ge, for which layers as thin as 5 nm showed perpendicular magnetic anisotropy and reasonably square magnetic hysteresis loops. Ultra-thin films (˜1 nm thick) of these materials that exhibit large PMA grown on chemical templating layer (CTL) required use of single crystalline substrates such as MgO(100) or use of MgO seed layers on Si/SiO2 substrates. Such single crystalline substrates or use of MgO as part of the seed layer are not useful for STT-MRAM applications in which the MTJs must be deposited on wires that are formed in today's CMOS based technologies from polycrystalline copper that may be covered with other layers that are also polycrystalline or amorphous. In order to be able to use ultra-thin tetragonal Heusler compounds as magnetic electrodes switchable by STT for MRAM a method for forming these compounds on amorphous or polycrystalline substrates or layers is needed.